Experiments are proposed to study functional heterogeneity in the phenotypically heterogeneous population of cells capable of mediating lymphokine-activated killer (LAK) activity in humans. The availability of large numbers of purified LAK cells from patients receiving interleukin 2 (IL-2) provides a unique resource for accomplishing these studies. The long-term objectives of this project are 1) to identify the cellular mechanisms responsible for LAK activity, 2) to characterize the mechanisms by which IL-2 mediates its effects on LAK precursor and effector cells in vivo, and 3) to utilize these experimental observations for the development of clinical trials that utilize purified or enriched populations of optimally-stimulated LAK cells. The first experimental aim is to characterize the functional heterogeneity of distinct subpopulations of human LAK effector cells generated in vivo and in vitro. Single cell assays of LAK-mediated cytolysis will be performed on phenotypically-purified LAK cells in order to assess target binding specificity and affinity, and rates of cytolysis among LAK effector cell subsets. Susceptibility to antibody and enzymatic treatments will be used to identify the role of various surface determinants in target cell recognition and lysis by purified LAK cell subsets. Modulation of LAK effector activity by cytokines other than IL-2 will be characterized on distinct LAK cell subsets. Ultrastructural studies will be performed to determine whether LAK killing potency is associated with qualitative and quantitative cytolytic granule content. The second experimental aim will characterize the acquisition of LAK cytolytic functions, surface markers, and ultrastructural characteristics by natural killer (NK) cells and phenotypically distinct LAK precursor cells. The relationship between NK cells and LAK cells will be examined using novel cell sorting techniques which will isolate NK cells based on their functional activity. Functional comparisons between activated NK and LAK activities will be performed to identify unique cellular traits associated with LAK activity. The effects of cytokines other than IL-2 or the development of LAK cells from precursors will be evaluated on phenotypically distinct LAKp subsets. These studies will provide considerable new information regarding the LAK cell phenomenon and will have a significant impact on the improvement of cancer therapy using IL-2 or LAK cells.